Speed Testing — iperf3 & friends

~15 min · iperf3, speedtest, throughput, 10gbe

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Two tools, two scenarios

Network speed has two distinct measurements. LAN throughput — how fast can two devices on your network actually talk? Use iperf3. Internet bandwidth — how much can you download from a public server? Use speedtest-cli or just curl a known-fast file.

iperf3 — the LAN throughput truth

iperf3 needs both ends. Run it as a server on one machine (iperf3 -s), as a client on another (iperf3 -c server-ip). It pumps as much data as the path can carry and reports the rate. This is how you confirm that your 10GbE actually works as 10GbE.

Code

iperf3 — LAN throughput·bash

# Install
brew install iperf3

# On the server
iperf3 -s

# On the client (the one running the test)
iperf3 -c 192.168.1.100

# Multiple parallel streams — saturates 10GbE much better
iperf3 -c 192.168.1.100 -P 4

# UDP test at a target rate
iperf3 -c 192.168.1.100 -u -b 10G

# Reverse — server pushes to client
iperf3 -c 192.168.1.100 -R

# Long test (60 seconds default → custom)
iperf3 -c 192.168.1.100 -t 30

Internet speed·bash

# speedtest-cli (Ookla via Python)
brew install speedtest-cli
speedtest-cli

# Or curl a known-fast file with timing
curl -o /dev/null \
  -w "download: %{speed_download} bytes/s\nsize: %{size_download} bytes\n" \
  https://speed.hetzner.de/100MB.bin

External links

Exercise

Pick two machines on your fastest network. Install iperf3 on both. Run iperf3 -s on one, iperf3 -c that-ip -P 4 on the other for 30 seconds. Note the result. If you have 10GbE and see less than 5 Gbps, the network is misconfigured somewhere — Track 7 will help diagnose. If you see expected speed, you've just verified your network's actual ceiling.